The present invention relates to satellite-navigation receivers and systems, and more particularly to devices and methods for operation in very-low signal-strength areas, and those that can initialize and output position solutions very quickly.
Global positioning system (GPS) receivers use signals received from typically three or more earth-orbiting satellites to determine navigational data such as position and velocity. GPS signals are available worldwide at no cost and are now being routinely used to determine the location of automobiles to within one city block, or better. Dual-frequency carrier GPS receivers typically track a pair of radio carriers, L1 and L2, associated with the GPS satellites to generate accumulated delta-range measurements (ADR) from P-code modulation on those carrier frequencies and at the same time track L1 C/A-code to generate code phase measurements. Carrier frequency L1 is allocated to 1575.42 MHz and carrier frequency L2 is positioned at 1227.78 MHz. Less expensive receivers tune only one carrier frequency, and therefore do not have adequate information to compute the local troposheric and ionospheric signal-propagation delays that appear as position errors. At such frequencies, radio carrier signals travel by line-of-sight. Thus buildings, mountains and the horizon can block reception, and multipath reflections can interfere with good reception.
Each one of the constellation of GPS satellites in orbit about the earth transmits one of thirty-two unique identifying codes in a code-division multiple access (CDMA) arrangement. This allows all of the many GPS satellites to transmit in spread spectrum mode at the same frequency, plus or minus a Doppler frequency shift of that frequency as results from the satellite""s relative velocity. Particular satellites are sorted out of a resulting jumble of signals and noise by correlating a 1023 xe2x80x9cchipxe2x80x9d code to one of the thirty-two pseudo random number (PRN) sequence codes that are preassigned to individual GPS satellites. These codes are not necessarily being transmitted in phase with one another. Therefore, xe2x80x9cfindingxe2x80x9d a GPS satellite initially involves searching various carrier frequencies, to account for Doppler frequency shift and oscillator inaccuracies, and searching for a code match, using 1023 different code phases and twenty or more possible correlation code templates.
In large cities with many tall buildings, one or more of the GPS satellites being tracked by a particular receiver, may be temporarily blocked. In some situations, such blockage can prevent all the overhead GPS satellites from being tracked and such outages can last for several minutes. GPS signals also become unavailable to vehicles moving through underground or underwater tunnels.
At least one conventional GPS five-channel receiver directs all of its channels to focus on one satellite at initial turn-on. This addresses the problem of satellite signal frequency uncertainty that exists due to Doppler effects and local oscillator inaccuracies in the receiver. A search for a particular satellite in the apparent-Doppler frequency spectrum is conducted in parallel. For example, by segmenting the possible Doppler frequency spectrum into as many segments as there are receiver channels. Then appointing each of the several receiver channels to attend to a search within a respective segment.
The single largest uncertainty stems from the random frequencies possible from typical local oscillators at start-up. Therefore, the apparent-Doppler frequency is known only within wide search boundaries. Knowing the actual Doppler frequency is not much help, because the local oscillator can be so far off nominal on its own.
From the user""s standpoint, at least two operational characteristics of prior art GPS receivers interfere with complete satisfaction. Such conventional receivers often quit working indoors because the buildings reduce the local signal field level to less than the receiver""s maximum sensitivity. And, most receivers take a very long time to produce a position solution from a cold start.
Intensive calculations in GPS receivers have necessitated high clock speeds and lots of expensive storage memory. These, in turn, demand expensive and comprehensive hardware. Manufacturers and users alike would appreciate lighter and thinner navigation solutions that could use inexpensive platforms or share pre-existing platforms for other applications.
The Internet also represents a way for individual GPS receivers to monitor differential correction data in their areas and to off-load calculation-intensive tasks on regional webservers that have high performance processors.
SnapTrack, Inc., (San Jose, Calif.) is a commercial supplier of wireless assisted GPS (WAG) systems. Time, frequency, and approximate location data are extracted from a wireless network to assist GPS-signal processing in a navigation receiver. Such technology is described in a number of United States Patents assigned to SnapTrack, including: U.S. Pat. Nos. 5,945,944; 5,663,734; 5,781,156; 5,825,327; 5,831,574; 5,841,396; 5,812,087; 5,874,914; 5,884,214; etc. Also see, U.S. Pat. No. 6,078,290.
It is therefore an object of the present invention to provide a satellite-navigation receiver that can work indoors with extremely low signal strength levels.
It is another object of the present invention to provide a satellite-navigation receiver that produces position solutions rapidly after each cold start.
It is a further object of the present invention to provide a satellite-navigation system that is inexpensive.
It is a still further object of the present invention to provide a satellite-navigation system that interfaces with the Internet.
It is an object of the present invention to provide a system that can be assisted in improved accuracy and performance via time-difference and frequency-difference information distributed in a network.
Briefly, a GPS-positioning system embodiment of the present invention comprises a system-on-chip receiver, a network-client and a webserver. The webserver can support simple-observer platform mode where the system-on-chip receiver merely passes through information, or it can support a half-autonomous mode where the system-on-chip receiver is able to compute its position as long as fifteen minutes after its last contact with the webserver. The webserver includes its own local GPS-constellation measurement platforms to build a database of GPS-measurement errors, ephemeris, almanac, and other satellite-data messages. Information from poor-performing satellites with problems is filtered out. The network client is relieved of traditional computation-intensive tasks by using integer arithmetic and communicating binary numbers rather than floating-point or using transcendental math functions. In one mode, position fixes are provided by the webserver from data passed from the network client. The system-on-chip receiver includes circuits to boost receiver sensitivity and search speed.
An advantage of the present invention is that a system and method are provided that reduce the costs of user navigation equipment.
Another advantage of the present invention is that a system and method are provided that improve sensitivity and time-to-first-fix enough for urban canyon and indoor use.
A further advantage of the present invention is that a system and method are provided that makes higher level database services to be offered on the Internet which are related to real-time and historical user position fixes.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.